It is well known in the art to produce articles of commerce from thermoplastic materials by means of extrusion blown molding, extrusion injection molding, or simple extrusion of thermoplastic materials. These materials typically consist of low density polyethylene, high density polyethylene, polypropylene, polyvinyl chloride, acrylonitrile butadiene styrene copolymer, nylon, polycarbonate, polybutylene terephthalate, polyethylene terephthalate, polyphenylene sulfide, styrene-acrylonitrile, thermoplastic elastomers, as well as other polymers, copolymers and mixtures of these materials.
In actual practice, these thermoplastic polymers (TPs) must first be converted from a powdered, granular or pelletized form into a continuous stream of melted TP which then may be rolled into a desired shape, injected into a mold of the desired shade, or formed into a cylinder or parison which may be further expanded by air to conform to the inside surfaces of a blow molding mold cavity. The conversion of TPs into the required continuous melt stream is usually accomplished through the use of a heated barrel screw extruder.
The typical screw extruder basically consists of a barrel and screw, and also contains a feed section and a discharge port. The barrel consists of a long heavy-walled tube of steel which contains, as its inside surface, a hard wear-resistant alloy, and the bore of the tube is ground to very close tolerances. The complete construction of the barrel is such that it is capable of handling very high pressures, in the range of 1,000-10,000 psi, without distorting or rupturing.
The outside of the barrel is usually fitted with heating or cooling units along its length, and these units are configured in zones along the length of the barrel, such that various sections of the barrel may independently be heated or cooled as desired.
An opening at the feed end of the barrel serves as an introduction point for powdered, granular, or pelletized TPs into the screw, as well as for the introduction of coloring agents, fillers etc. which may be included as desired. To avoid premature melting of the TP feed, the area or zone around the feed section may be cooled by means of a cooling unit.
The barrel is fitted with a close fitting internal progressive screw with helical flights, and by rotating this screw in the proper direction, material is advanced in the barrel from the feed end toward the discharge end. The screw rotating in the barrel acts, in effect, like a pump. The screw design is such that material advancing through the barrel is subjected to ever increasing pressure as the cavity volume created between the screw and the barrel diminishes as viewed along the length of the assembly from the feed end to the discharge end. In actual practice, a typical screw may consist of three distinct sections: a feed section which serves to push material into the barrel; a compression section which exerts ever increasing pressure and a high amount of shear on the TP along its length; and a metering section which serves to complete the homogenization and melting of the TP and to force the material out of the assembly.
Actual melting of the TP is usually caused by both heat applied to the barrel by the heating units, as well as by the heat developed in the TP due to the mechanical action applied to it by the screw, since a certain amount of slippage or bypass of the melted TP occurs along the screw flights and between the screw and the barrel. This slippage and bypass also serves to homogenize the TP and any adjuncts, such as coloring agents, added to the feed section of the unit.
This slippage and bypass, although required for efficient TP processing, causes unwanted problems, however, when finished product colors and/or the type of TP material must be changed to meet production requirements. The constant back feeding and blending which occurs in the unit, with subsequent progressive dilution of the old color or feed stock with the new color or feed stock, necessitates the production of large amounts of undesirable or unusable material until the original color or TP is cleared completely from the unit, and only the second color or TP exits from the unit in its unadulterated state. Additionally, during normal production, degraded materials, such as carbonized resin particles, form in the apparatus and periodically appear in finished parts, and these materials must also be cleaned from the equipment.
In an effort to speed up this cleaning or change over process, compounds known as purging compounds (PCs) are employed. There are two basic types of PCs, physical and chemical.
Physical PCs (PPCs) are merely thermoplastics, occasionally containing abrasive materials such as diatomaceous earth, which may have a higher melt point than that of the material being purged from the equipment. Thus they operate by attempting to physically push the old material out of the equipment. In most cases they are marginally effective at best, and, if abrasive, can cause premature wearing of the screw, barrel and associated equipment if used on a regular basis. A PPC can also consist simply of a clear or neutral thermoplastic which only acts as an indicator to tell the operator when the original color has been purged from the equipment. In any case, large amounts of material must be run through the equipment, since the constant mixing and blending action of the screw tends to successively dilute the original color with the PPC rather than just push it through and out of the equipment. In actual practice, the new color is sometimes simply charged to the screw, and the process is run with the new color until all traces of the old color have disappeared. This may take up to several hours, and may result in hundreds of pounds of scrap. In extreme cases, the old color still occasionally appears in manufactured parts several days after changeover.
In any event, PPCs do not usually remove material from dead spots in the system, such as occur in valves and dies, and degraded materials such as carbonized particles of polymer usually remain in the equipment to exit at a later time resulting in manufactured parts which do not meet specifications.
Another type of purging compound, the chemical purging compound (CPC), attacks the problem in most cases by attempting to break down plastic residues in the equipment. These compounds typically contain thermoplastic resins, organic and inorganic salts, and inert materials. Typically their use may require that the feed to the screw be cut off, and the equipment then run until material ceases to exit from the discharge end. A quantity of neutral color thermoplastic is then charged to the screw and allowed to run out in the same manner. The system is then filled with CPC, and as soon as it begins to exit, the screw rotation is stopped, and with heat continually applied to the barrel, the compound is left in the equipment for a period of time, usually 15 minutes or more, to complete the reaction. In some cases, the temperature of the barrel heating units is increased, causing the barrel temperature to rise. After the reaction is deemed to be complete, the barrel temperature is lowered to its original state and the screw is restarted, causing the CPC to exit. The new desired color is then introduced into the screw feed and the remainder of the CPC is purged by means of the new color.
It is not unheard of for a chemical purge to take in excess of two hours to complete, from start to finish, especially if the barrel temperature is to be raised and then lowered. Many CPCs also tend to emit organic vapors and gasses as they work, and from an ecological and safety standpoint, leave much to be desired.
Another type of PC, the liquid PC, is also occasionally employed. These liquids may be aqueous in nature, and although usually inexpensive, are prone to slippage, wherein the friction between entire thermoplastic mass and the screw and barrel is reduced to the point where the assembly no longer acts like a pump, and the mass ceases to be forced under high pressure through the unit. Additionally, the aqueous component of these liquid PCs does not combine with the resins in an efficient and homogeneous manner within the apparatus.
As a testament to the ineffectiveness of the PCs currently available, one accepted and standard practice employed in color or material changeovers is the complete disassembly of the equipment followed by manual cleaning and subsequent reassembly. This process is both time consuming and labor intensive, and can result in equipment down time in excess of 16 hours. And yet, as costly as this procedure may seem, many processors currently consider this cleaning method to be the best available option, in light of the ineffectiveness of presently available purging compounds.
It is believed that this ineffectiveness of currently available PCs is due to the fact that the forward or leading surfaces of the screw flights are, during operation, subjected to high material drag forces which tend to allow the PCs to scrub these surfaces free of old color and material. The trailing surfaces of the screw flights, however, are not subjected to drag forces as high as those which occur on the leading surfaces, and it is this thermoplastic material and carbonized and other degraded material attached to these trailing surfaces that currently available PCs fail to remove effectively. Dead spots in the equipment, as in valve and die assemblies, wherein the velocity of molten thermoplastic materials under normal operation is lower than the velocity found in most parts of the system, also tend to cause a problem for currently available PCs, in that material in these dead spots tends to remain in place during purging.
Some commercially available purging products attempt to overcome this problem by requiring that the apparatus be filled with purging compound, the temperature of the apparatus be raised significantly, and the purging compound be allowed to remain in contact with the internal surfaces of the apparatus for an extended time period, e.g. 10 to 30 minutes. The temperature of the apparatus must then be lowered to the original processing temperature. Thus the time required to raise and lower the apparatus temperature, as well as the contact time required, can be significant and highly undesirable in a production environment, especially if a second purge is required.
A number of purging compounds are currently being sold commercially, and references may be cited pertaining to extruder purging compounds. In published Japanese patent application JP5269754, a purging compound is described which consists of a thermoplastic resin having particular melt flow characteristics. U.S. Pat. No. 5,236,514 describes a purging composition containing both a matrix and a carrier resin of specific compositions along with a phosphate ester or metal salt of a carboxylic acid and an abrasive such as diatomaceous earth. In published international application WO 8908017, a compound for purging is disclosed which contains at least two different kinds of hard particulate material having different average particle radii along with a specific thermoplastic resin composition and a surfactant, and in which the harder particulate materials act as abrasives. In published Japanese applications JP62176817 and JP62117712, purging compositions are described which consist of an aromatic polyester resin having a higher melt viscosity than the thermoplastic resin being purged, and also contain water or a compound containing water of crystallization. In all of the above cases, as well as in the case of currently available purging compounds, attempts are made to scrape the material from the apparatus, remove it with a higher viscosity thermoplastic material, or chemically lower the viscosity of, or break down the resin being purged. These purging methods and compounds are not effective in addressing the root of the purging problem, that is, removal of resin from the lower pressure areas of the screw, i.e. the trailing edges of screw flights, and dead, or low velocity areas, in the equipment, and they require long periods of time to accomplish their stated purposes.